#include <iostream>
#include <string>

using namespace std;

int main() 
{
	string str;
	while(cin >> str) {
		int count[256] = {0}, i = 0;
		
		for (i = 0; i < str.length(); ++i)
			count[str[i]]++;

		for (i = 0; i < str.length(); ++i) {
			if (count[str[i]] == 1) {
				cout << str[i] << endl;
				break;
			}
		}
		
		if (i == str.length())
			cout << "no characters occurring only once!" << endl;
	}

	return 0;
}
